Conventional dual-band bandpass filters, which have two pass bands, are generally classified into two types according to structure.
One type is a filter 200 shown in FIG. 50 that is composed of a plurality of (three, in this example) dual-band bandpass resonators Q1, Q2 and Q3, which resonate at two frequencies, cascaded to each other and input/output ports P1 and P2 coupled to the opposite ends of the cascade (see the non-patent literature 1, for example). For the filter 200, the dual-band bandpass resonators Q1 and Q3 coupled to the input/output ports P1 and P2 at the opposite ends of the cascade need to have a coupling part having a configuration and dimensions that provide a desired center frequency and bandwidth both in the two bands.
The other type is a filter 300 shown in FIG. 51 that is composed of a plurality of transmission lines T1 to T9 having different impedances and line lengths connected to each other at the ends thereof (see the non-patent literature 2, for example). For the filter 300, the characteristics as the dual-band bandpass filter is achieved by determining the characteristic impedance and length of each transmission line of the filter based on an equivalent circuit theory using lumped parameter elements.    Non-patent literature 1: S. Sun, L. Zhu, “Novel Design of Microstrip Bandpass Filters with a Controllable Dual-Passband Response: Description and Implementation,” IEICE Trans. Electron, vol. E89-C, no. 2, pp. 197-202, February 2006    Non-patent literature 2: X. Guan, Z. Ma, P. Cai, Y. Kobayashi, T. Anada, and G. Hagiwara, “Synthesizing Microstrip Dual-Band Bandpass Filters Using Frequency Transformation and Circuit Conversion Technique,” IEICE Trans. Electron, vol. E89-C, no. 4, pp. 495-502, April 2006